With limited cycling experience, I joined a group ride last year. I had never drafted, had never ridden the course, and knew one guy in the group. Worse I showed up on a hybrid bike (Specialized Sirrus) to ride with a group of full kit road guys. They have been great, I ride with them every weekend, and here is what I have learned:
Conserve your energy from the minute you start. If they seem to be going insanely slow, stay with them as they are probably prepping for something. Most of the time we start with a few miles of warm up. If that pace is skipped or insane, I know I am in trouble.
Tell the people around you that it is your first group ride, and you have no drafting skills. Someone may very well offer to work with you, and will usually work in the very back of the pack. Here it is called "the cat bird seat". You won't pull, and you won't screw up anyone behind you with an inconsistent pull.
To learn to draft, do not get on someone's wheel immediately. Practice pacing 3 feet back and match speed, then move in closer in stages, to build comfort. Don't coast, soft pedal if you are going too fast. When you are close enough you will feel it, and will almost feel pulled forward. Watch for it.
The hills separate the pack, and quickly. Have a strategy for the hills, especially if you have ridden the course. Know the ones you can push and the ones you can't. Use your conserved energy wisely.
There is no prize for finishing first, even though someone is always going to race.
Our group averages 19 ish over 50 miles, but that includes some long sections going 26 to 28 and a rolling 1 mile all out sprint that can run from 29 to 33. Natural selection may make the one group into three by the time it is over. It happens.
Never cross the yellow line. It's like crossing the beams in Ghost Busters. If someone calls out "Car Back" then move right safely, and let traffic move on. If you stake out the cat bird seat, shout out "Car Back" so people can hear it.
Be safe. Don't ride with a group if they aren't
If insecure, leave the bling at home on the first ride. Rather than the kit I own, I was black shorts, long undies top and a soccer jersey for first ride. They called me "Kid on a Mountain Bike" until I held onto a long climb with one of the strongest guys in the group. Then I was "Kid on a Mountain Bike with really strong legs".